Ethylene-vinyl alcohol based copolymer (hereinafter also referred to as “EVOH”) is a thermoplastic resin that is excellent in gas barrier properties, oil resistance, antistatic properties, mechanical strength, and the like. EVOH is widely used for various packaging materials such as films, sheets, and containers. To be processed into various molded products, EVOH is molded in a molten state by, for example, extrusion molding or injection molding. When molding EVOH in a molten state, the molding temperature usually needs to be 200° C. or higher. For this reason, thermal degradation tends to occur, and the molded products may suffer from fish eyes or hard spots, which can be a cause of degradation in product quality.
It has been known that the long-term run stability of EVOH is improved by adding to the EVOH an organic carboxylic acid, an alkali metal salt, an alkaline earth metal salt, a phosphate, or the like (for example, WO99/05213 and JP 2001-164059A). However, such a method, which seems to be a stopgap measure, has not resolved the problem fundamentally nor achieved an improvement in the long-term run stability to a sufficient degree.
Thus, conventionally, it has long remained as an unsolved problem to suppress quality degradation of EVOH due to fish eyes and hart spots formed in a high-temperature molding process.